For automotive interior trim, a vinyl chloride resin has been widely used in the past. Since the vinyl chloride resin is not only excellent in molding processability into parts but also relatively inexpensive, it has been often used for skin materials of interior parts, such as instrument panel, door and ceiling, handle materials, knobs of gearshifts, various grips, etc. In recent years, however, it has been pointed out that the vinyl chloride resin has a possibility of becoming a cause of a harmful gas when it is incinerated, and in the above parts, replacement of the vinyl chloride resin with other materials has become the main stream.
Especially in the automotive interior trim, most widely used as a substitute for the vinyl chloride resin is a thermoplastic elastomer. Since the thermoplastic elastomer is an excellent material from the viewpoints that it is lightweight, is excellent in moldability and processability into parts, can be readily recycled and generates no harmful gas when it is incinerated, uses of the thermoplastic elastomer have been further increased.
In the use applications to interior trim skins that are typical use applications of the thermoplastic elastomer, the thermoplastic elastomer is generally used for instrument panel skins, door skins, ceiling skins, console skins, etc. at present by processing a thermoplastic elastomer sheet, which is obtained by extrusion method or calendaring, through vacuum forming or stamping and then laminating the resulting product onto a substrate of foamed polyurethane or foamed polypropylene. As a method for such laminating, there is known a method of casting liquid urethane between a substrate and a thermoplastic elastomer or a method of integrally molding a substrate resin with a laminated sheet of a thermoplastic elastomer and foamed polypropylene through injection compression molding.
In recent years, a method in which a step of laminating foamed polyurethane or foamed polypropylene is omitted for the purpose of simplifying the process and a foamed thermoplastic elastomer is laminated onto a substrate resin itself has been proposed. In this method, however, there resides a problem that a soft feel required is not obtained because of insufficient foamability of the thermoplastic elastomer or good appearance is not obtained because of non-uniform foaming.
On the other hand, with regard to automotive materials, studies of foaming of a thermoplastic elastomer have been made from the viewpoint of weight lightening. Usual thermoplastic elastomers, such as olefin-based plastic components of patent literatures 1 and 2, are decomposed when they are dynamically heat-treated in the presence of an organic peroxide, and have poor tension during melting. Therefore, such thermoplastic elastomers have problems that they are liable to be defoamed, and even if a foam is obtained, its expansion ratio is at most about 1.5 times and surface roughening due to defoaming is marked. Moreover, there is another problem that if the amount of the organic peroxide added is reduced, heat resistance of the resulting molded article is lowered though the foamability is improved.
In patent literatures 3 and 4, a foam containing a polyolefin-based resin having a melt tension of a specific value or more has been proposed, and in a patent literature 5, a composition the longest relaxation time of which was limited has been proposed, but sufficient foamability has not been obtained.
In the patent literature 5, it has been proposed that by the use of a thermoplastic elastomer containing a propylene/α-olefin copolymer having a specific molecular weight distribution, a soft feel and appearance of good quality are obtained in injection molding. In such composition, however, sufficient foamability is not obtained, and therefore, a soft feel required is not obtained. Moreover, if the amount of a blowing agent is increased in order to sufficiently increase foamability, there is a disadvantage of deterioration of appearance though desired foamability is obtained.
In patent literatures 6 to 9, a plastic having a syndiotactic structure, addition of a high-molecular weight component, etc. have been proposed, but sufficient foamability has not been obtained.